TAS Reduction above crossover altitude

[Ledhead27]

Hi all,

Just been doing some research for an ATPL uni assignment and trying to get my head around the rate of TAS reduction once the crossover altitude has been reached/exceeded.

I understand that climbing up to the crossover altitude is conducted with a constant CAS (and increasing TAS) until reaching the crossover altitude, whereby constant Mach is used (therefore decreasing TAS).

However, above this crossover altitude does TAS reduce at the same rate as below the crossover altitude (if that made any sense). Also, if an aircraft reaches its cruise flight level where the TAS is lower, could the operator simply not cruise at the crossover altitude. The only issue I could think for doing this however is the proximity of other aircraft also cruising at the crossover altitude to achieve the highest TAS.

Any help would be most appreciated!

[donpizmeov]

It all depends on what happens with the temperature. For example, on an ISA day the machine I fly will TAS at 513kts at M0.85 at FL300 (crossover ALT). Keeping ISA we get we get 490ish at FL400 at same Mach speed, or 500ks at ISA plus 10.

Cost index is used to optimise cruise ALT. Disregarding winds aloft, A higher cost index will result in a lower cruise ALT, with faster TAS and higher fuel flow. A lower Cost index will increase the ALT to minimise fuel flow and result in a slower TAS.

[mcdhu]

So if you're in a hurry, go at FL290!
mcdhu

[ROKVIATOR]

Hi, as far as i am concerned, the climb is at constant IAS and then, you will change to constant MACH, climbing at a constant IAS will result in a MACH increase upon reaching a level in which you will change to constant MACH, as you may know from instrumentation a climb with constant mach will result in a reduction of IAS/TAS/EAS/CAS...

Some reasons not to fly at the optimum altitude may be more headwind or more tailwind at other altitude. Ant the reason of being above, below or at the optimum altitude is just fuel saving. I dont know if that makes sense for you or answer anything you were asking/wondering.

[framer]

The answer is money. It costs too much to cruise at the crossover level.

[LOMCEVAK]

NB that for a standard atmosphere, when climbing above the tropopause at constant Mach then TAS will be constant. The issue with cruising at the crossover altitude is that you need to consider the required RPM for range speed or LRC and that it may well be below the optimum for engine efficiency and so specific air range will be less than flying higher at the optimum altitude.